.TH std::ranges::uninitialized_fill 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::ranges::uninitialized_fill \- std::ranges::uninitialized_fill

.SH Synopsis
   Defined in header <memory>
   Call signature
   template< no-throw-forward-iterator I, no-throw-sentinel-for<I> S, class
   T >
                                                                            \fB(1)\fP (since
   requires std::constructible_from<std::iter_value_t<I>, const T&>             C++20)

   I uninitialized_fill( I first, S last, const T& x );
   template< no-throw-forward-range R, class T >

   requires std::constructible_from<ranges::range_value_t<R>, const T&>     \fB(2)\fP (since
   ranges::borrowed_iterator_t<R>                                               C++20)

   uninitialized_fill( R&& r, const T& x );

   1) Constructs \\(\\scriptsize N\\)N copies of the given value x in an uninitialized
   memory area, designated by the range [first, last), where \\(\\scriptsize N\\)N is
   ranges::distance(first, last).
   The function has the effect equivalent to:

 for (; first != last; ++first)
     ::new (static_cast<void*>(std::addressof(*first)))
         std::remove_reference_t<std::iter_reference_t<I>>(x);
 return first;

   If an exception is thrown during the initialization, the objects already constructed
   are destroyed in an unspecified order.
   2) Same as \fB(1)\fP, but uses r as the range, as if using ranges::begin(r) as first, and
   ranges::end(r) as last.

   The function-like entities described on this page are niebloids, that is:

     * Explicit template argument lists cannot be specified when calling any of them.
     * None of them are visible to argument-dependent lookup.
     * When any of them are found by normal unqualified lookup as the name to the left
       of the function-call operator, argument-dependent lookup is inhibited.

   In practice, they may be implemented as function objects, or with special compiler
   extensions.

.SH Parameters

   first, last - iterator-sentinel pair denoting the range of elements to initialize
   r           - the range of the elements to initialize
   value       - the value to construct the elements with

.SH Return value

   An iterator equal to last.

.SH Complexity

   \\(\\scriptsize\\mathcal{O}(N)\\)𝓞(N).

.SH Exceptions

   The exception thrown on construction of the elements in the destination range, if
   any.

.SH Notes

   An implementation may improve the efficiency of the ranges::uninitialized_fill, e.g.
   by using ranges::fill, if the value type of the output range is TrivialType.

.SH Possible implementation

   struct uninitialized_fill_fn
   {
       template<no-throw-forward-iterator I, no-throw-sentinel-for<I> S, class T>
       requires std::constructible_from<std::iter_value_t<I>, const T&>
       I operator()(I first, S last, const T& x) const
       {
           I rollback{first};
           try
           {
               for (; !(first == last); ++first)
                   ranges::construct_at(std::addressof(*first), x);
               return first;
           }
           catch (...)
           {
               // rollback: destroy constructed elements
               for (; rollback != first; ++rollback)
                   ranges::destroy_at(std::addressof(*rollback));
               throw;
           }
       }

       template<no-throw-forward-range R, class T>
       requires std::constructible_from<ranges::range_value_t<R>, const T&>
       ranges::borrowed_iterator_t<R>
       operator()(R&& r, const T& x) const
       {
           return (*this)(ranges::begin(r), ranges::end(r), x);
       }
   };

   inline constexpr uninitialized_fill_fn uninitialized_fill{};

.SH Example


// Run this code

 #include <iostream>
 #include <memory>
 #include <string>

 int main()
 {
     constexpr int n{4};
     alignas(alignof(std::string)) char out[n * sizeof(std::string)];

     try
     {
         auto first{reinterpret_cast<std::string*>(out)};
         auto last{first + n};
         std::ranges::uninitialized_fill(first, last, "▄▀▄▀▄▀▄▀");

         int count{1};
         for (auto it{first}; it != last; ++it)
             std::cout << count++ << ' ' << *it << '\\n';

         std::ranges::destroy(first, last);
     }
     catch(...)
     {
         std::cout << "Exception!\\n";
     }
 }

.SH Output:

 1 ▄▀▄▀▄▀▄▀
 2 ▄▀▄▀▄▀▄▀
 3 ▄▀▄▀▄▀▄▀
 4 ▄▀▄▀▄▀▄▀

   Defect reports

   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.

      DR    Applied to              Behavior as published              Correct behavior
   LWG 3870 C++20      this algorithm might create objects on a const  kept disallowed
                       storage

.SH See also

   ranges::uninitialized_fill_n copies an object to an uninitialized area of memory,
   (C++20)                      defined by a start and a count
                                (niebloid)
                                copies an object to an uninitialized area of memory,
   uninitialized_fill           defined by a range
                                \fI(function template)\fP
